Windows for use in the doors of cooking stoves and other relatively low temperature heating chambers have conventionally been shaped from standard soda lime glasses, or where somewhat higher temperature capability was believed necessary, from borosilicate glasses, e.g., PYREX brand products marketed by Corning Glass Works, Corning, N.Y. With the recent dramatic increase in the cost of heating oil and natural gas, there has been a great upsurge in the use of coal and wood stoves for heating rooms or entire homes and other relatively small buildings. Most generally, those stoves have a window in the door or other area to permit the user to observe the burning taking place inside.
Coal and wood stoves customarily operate at temperatures higher than those encountered in the traditional gas-fired or electric cooking stove, and the atmosphere generated therein through the combustion of coal and/or wood is normally more chemically corrosive than that produced in cooking food. Furthermore, because of the manner in which the coal and/or wood is charged into the stove and the possible subsequent movement thereof in the stove during burning, the viewing window may occasionally be exposed to impacts and other physical abuse.
Windows for use in those stoves have been fabricated from borosilicate glasses but such have been subject to cracking both from thermal and mechanical causes. Coal and wood stoves may reach temperatures in excess of 600.degree. C. when in operation. The conventional borosilicate glass exhibits a strain point of about 510.degree. C. and a coefficient of thermal expansion of about 32.5.times.10.sup.-7 /.degree.C. over the range of 0.degree.-300.degree. C.
Windows for use in those stoves have also been prepared from 96% silica glass, e.g., VYCOR brand products marketed by Corning Glass Works. Such glasses deomonstrate strain points of about 829.degree. C. and coefficients of thermal expansion of about 8.times.10.sup.-7 /.degree.C. over the range of 0.degree.-300.degree. C. Those glasses have proven to be more satisfactory for stove windows than the borosilicate compositions for two reasons. First, they are essentially immune to cracking resulting from thermal causes at the temperatures encountered. Second, because they are essentially free from alkali metals and are substantially pure silica, they are very highly resistant to attack by the chemical components in the stove atmosphere. However, such glasses are considerably more expensive than borosilicate compositions and, in like manner to the borosilicate glasses, are subject to damage through impact and other physical abuse.
In an effort to provide a transparent window with improved mechanical properties, while exhibiting the necessary thermal shock resistance and refractoriness, transparent glass-ceramic compositions have been investigated. Transparent glass-ceramic articles are well-known to the art; the classic study thereof being "Tansparent Glass-Ceramics", G. H. Beall and D. A. Duke, Journal of Materials Science, 4, pages 340-352 (1969). Those authors explained that glass-ceramic products will be transparent to the eye when the crystals present therein are substantially smaller than the wavelength of visible light or the birefringence within the crystals and the refractive index difference between the crystals and the residual glass are very small. The writers discussed three general composition areas in the aluminosilicate glass-ceramic system wherein highly transparent articles can be produced, one of those areas being the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 field wherein .beta.-quartz or .beta.-eucryptite solid solution, as such as been variously termed, comprises the predominant crystal phase. U.S. Pat. Nos. 3,157,522, 3,241,985, 3,252,811, 3,282,712, 3,484,327, 3,499,773, and 3,788,865 are illustrative of specific composition areas within the Li.sub.2 O-Al.sub.2 O.sub.3 -SiO.sub.2 system suitable for the preparation of such transparent products. Because of the inherent high crystallinity of glass-ceramic articles, the mechanical strengths exhibited thereby are customarily appreciably greater than glass bodies; for example, moduli of rupture twice that manifested by the present glass. .beta.-quartz or .beta.-eucryptite solid solution crystals can demonstrate coefficients of thermal expansion over the range of 0.degree.-300.degree. C. of 0 or less. Consequently, a highly crystalline glass-ceramic containing such crystals as the predominant crystal phase will exhibit excellent thermal shock resistance. Finally, such a glass-ceramic will display high refractoriness because of the high melting point of the crystals, coupled with the fact that the small proportion of residual glass is of a high aluminosilicate composition which has a high softening point.
U.S. Pat. No. 3,148,994 discloses glass-ceramic compositions containing fluoride as a mandatory constituent which are notable for their high mechanical strength. The exact mechanism through which fluoride imparts the improved strength to the bodies was not understood, but the presence of at least 0.1% and up to 1.2% by weight (0.2-0.5% optimum), as analyzed, was clearly shown to have a favorable effect upon the modulus of rupture of the final product. The base compositions described in that patent consisted essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 65-75 Li.sub.2 O 1-4 MgO 1-4 Al.sub.2 O.sub.3 15-25 ZnO 0.5-2.0 Na.sub.2 O and/or K.sub.2 O 0.1-2.0 TiO.sub.2 3-6 F 0.1-1.2 ______________________________________
Where the precursor glass bodies having compositions within those ranges are crystallized in situ via heat treating at temperatures no higher than about 850.degree. C., transparent glass-ceramic articles are produced wherein .beta.-quartz solid solution constitutes the predominant crystal phase. A minor amount of rutile crystallization may also be present.
Windows for coal and wood stoves fabricated from glass-ceramic sheet having the following composition, as analyzed in weight precent, have been marketed by Corning Glass Works under Code 9618. The glass-ceramic is transparent and contains .beta.-quartz solid solution as essentially the sole crystal phase present.
______________________________________ SiO.sub.2 67.4 Al.sub.2 O.sub.3 20.4 LiO.sub.2 O 3.5 Na.sub.2 O 0.2 K.sub.2 O 0.2 ZnO 1.2 MgO 1.6 TiO.sub.2 4.8 As.sub.2 O.sub.3 0.4 Fe.sub.2 O.sub.3 0.03 F 0.2 ______________________________________
The coefficient of thermal expansion (0.degree.-300.degree. C.) exhibited by the glass-ceramic is about 2.3.times.10.sup.-7 /.degree.C.; hence, the material is very resistant to thermal shock.
However, experience had indicated that Code 9618 is subject to chemical attack after relatively short term exposure to the atmopshere of a coal or wood buring stove. Thus, the surface of the glass-ceramic becomes rough and crazing is developed therein.
U.S. Pat. No. 4,018,612 discloses glass-ceramic articles of very high transparency to visible light, excellent transmission to infrared radiation, and coefficients of thermal expansion (room temperature-600.degree. C.) of less than 10. The articles contain .beta.-quartz solid solution as the predominant crystal phase and are formed by exposing precursor glasses to heat treatments between about 850.degree.-950.degree. C., the precursor glasses consisting essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 67-70 Li.sub.2 O 2.5-3.5 MgO 1.5-2.5 Al.sub.2 O.sub.3 17.75-20 ZnO 1-2 TiO.sub.2 2-4.5 ZrO.sub.2 1-2 BaO 0-2 ______________________________________
Alkali metal oxides other than Li.sub.2 O, alkaline earth metal oxides other than MgO and BaO, and B.sub.2 O.sub.3 are essentially absent from the composition. Whereas the compositions of U.S. Pat. No. 3,148,994 utilize TiO.sub.2 alone as the nucleating agent, nucleation of the above glasses is promoted by a combination of TiO.sub.2 +ZrO.sub.2.
Th products of U.S. Pat. No. 4,018,612 exhibit optical and mechanical characteristics strongly recommending their application as windows for coal and wood burning stoves. Unfortunately, however, they are also subject to chemical attack resulting from exposure to the fumes present in such stoves.
Accordingly, the primary objective of the instant invention is to provide a method for inhibiting the susceptibility of the glass-ceramics of U.S. Pat. No. 4,108,612 and of fluoride-containing glass-ceramic bodies of the type disclosed in U.S. Pat. No. 3,148,994 and exemplified by Code 9618 to attack by the atmospheres generated in coal and wood burning stoves.